Studies on the biochemical and genetic characteristics of a key enzyme of carbohydrate metabolism, triosephosphate isomerase (TIM), in trophozoites of Giardia lamblia, were continued this year. In collaboration with Dr. Levenbook, purification of the enzyme was undertaken. Ammonium sulfate fractionations followed by chromatographic separation of the enzyme from other soluble proteins resulted in a 100- fold enrichment of TIM activity. Chromatography revealed that the parasite enzyme has properties which differ markedly from mammalian and trypanosome counterparts. In collaboration with Dr. Mowatt, transfection experiments with TIM gene from G.lamblia and a TIM chromosomal deletion mutant of Escherichia coli showed that TIM activity was now expressed by the mutant bacterium. Complementation with TIM genes suggests the general applicability of the method of cloning other genes for which E. coli mutants are available.